Water-absorbent resin particles are now widely used in various fields such as sanitary items (e.g., disposable diapers, feminine hygiene products), daily products (e.g., pet sheets), agricultural and horticultural materials (e.g., water-retaining materials, soil conditioners), and industrial materials (e.g., water-blocking materials for cables, dewfall preventing materials). Known examples of water-absorbent resin particles used in such applications include: hydrolyzed starch-acrylonitrile graft copolymers, neutralized starch-acrylic acid graft copolymers, saponified vinyl acetate-acrylic acid ester copolymers, and partially neutralized polyacrylic acid. Commonly required properties of water-absorbent resin particles include high water absorption, an excellent water absorption rate, high swelling capacity, and an appropriate median particle size in accordance with applications.
Among these, water blocking materials for cables are formed of two or more liquid-permeable sheets and water-absorbent resin particles that are fixed between the sheets, if needed, using an adhesive and the like. The demand for such water blocking materials has increased with development of electrical industry and communication industry. Water blocking materials for cables are used to wrap the cores of cables such as power cables and optical communication cables, and thereby the cores are protected. Then, the outside of the water blocking materials is covered with materials such as rubber. Thus, cables are formed. If the outer materials of cables such as power cables and optical communication cables are deteriorated to produce cracks therein and moisture enters through the cracks and reaches the cores of the cables, reduction in electric power and communication noise may be caused. The water blocking materials prevent such problems. The water blocking materials absorb such moisture and swell to increase the pressure in the cables, and thereby moisture is prevented from reaching the cores of the cables.
A water-absorbent resin used in water-blocking materials for power cables and communication cables is desired to prevent water immersion from the outside due to damage in cables at an early stage and to maintain the water-blocking effect for a long time. In addition, the water-absorbent resin is also desired to be produced efficiently and to be handled easily in the powder form during the production. Accordingly, for the purpose of realizing these performances, water-absorbent resin particles used in water-blocking materials are required to show high swelling capacity and a high water absorption rate, and to have an appropriate particle size that allows fine handleability.
A method for improving the swelling capacity of water-absorbent resin particles includes controlling the crosslink density of water-absorbent resin particles. For example, a method has been disclosed in which an acrylic acid/acrylate aqueous solution is subjected to reversed phase suspension polymerization in the coexistence of a surfactant having an HLB of 8 to 12, and a crosslinking agent is added thereto (immediately after the polymerization) for a crosslinking reaction (see Patent Literature 1).
Disclosed as a method for producing water-absorbent resin particles which show high water absorption and swelling capacity and which have a small particle size, for example, is reversed phase suspension polymerization of a water-soluble ethylenically unsaturated monomer in which, after first polymerization, a water-soluble ethylenically unsaturated monomer for second polymerization is added to a hydrocarbon solvent containing a surfactant and/or a polymer protective colloid dissolved therein and second polymerization is carried out (see Patent Literature 2).